Many waste waters, ground waters and process streams contain low solubility, volatile, organic contaminants, such as halogenated hydrocarbons and aromatic compounds. In the United States, about 9 billions kilograms of chemicals are accidentally or purposefully discharged into surface waters, underground wells and waste water treatment plants each year. Of this amount, about 2 billion kilograms are organic solvents. More than 50% of these solvents (about 1 billion kilograms) are discharged into aqueous streams at a concentration of less than 1%. Accordingly, there is a significant need to develop and improve separation processes and devices to be used to remove dilute organic solvents from aqueous waste streams.
Separation processes generally require preferential transfer of a component from a first phase, where the component is present in dilute form, to a second phase, where the component is present in a much more concentrated form. Separation processes are either an equilibrium process or a rate process. In an equilibrium process, a component in the first phase will distribute between two or more phases until satisfying an equilibrium condition (zero net interchange of components between the phases). Separation of a component between phases only occurs when there is an uneven distribution between phases. Absorption, for example, is an equilibrium process. In contrast, a rate process performs a separation based upon the rates at which a component moves from one phase into another phase. A rate process is a continuous process because the system is not allowed to come to equilibrium. Membrane pervaporation is a rate process.
There have been several techniques developed to address the need to remove volatile organic contaminants (VOCs) from aqueous streams. However, there are many disadvantages associated with such processes, including, for example, high energy costs, inability to conduct the process on a large scale, air emissions of organic vapor to transfer a water pollution problem into an air pollution problem, inability to recover the VOCs, and the problem of solid waste disposal of solid material containing the absorbed or adsorbed organic component.
One process that has been developed is a sorption process, which is an equilibrium process. In sorption (be it adsorption or absorption), the component in a first phase is attached to or is dissolved in sorbent material. The component is removed from the first phase until the sorbent material approaches equilibrium. The sorbent material is then removed and is either discarded (now creating a solid waste problem in the process of solving a liquid waste problem) or, if it is a solid such as carbon, is regenerated by adding copious amounts of heat or steam. Also, spent carbon must be incinerated to destroy adsorbed organic materials, because disposal of the carbon as solid waste may be considered as hazardous waste.
Membrane pervaporation is a rate separation process that utilizes thin membranes to separate components in a feed stream. Both volatile components and water permeate across the thin membrane. The rates at which volatile components in the feed stream and water permeate through a membrane (having a first side and a second side) must be in a different ratio to the concentration of the components in the feed stream in order for separation to occur. Pervaporation often requires that the entire waste water stream be heated to 60.degree. C. to 80.degree. C. in order to provide a high driving force for VOCs to pass through the membrane. This is an energy intensive process. The VOCs dissolve within the membrane matrix, and diffuse across the membrane to form a vaporous permeate stream on the second side of the membrane. The driving force for this diffusion is heating of the feed stream and applying a vacuum to the permeate stream.
Another technique is solvent extraction. This technique introduces another organic phase (the extractant) into the contaminated water that is immiscible with water but that can dissolve organic contaminants. The water and the extractant are mixed and then separated with the contaminants in the extractant. This process, however, utilizes a high volume of organic solvents. This can create disposal problems and requires further separation of the contaminant from the extractant.
Another technique that was in common use is air stripping. Air stripping removes VOCs from an aqueous phase and transfer them to air. Previously, the air was released into the atmosphere, making this procedure relatively inexpensive. Now, however, the air must be purified by activated carbon, or other techniques, making this procedure more expensive.
Steam stripping heats the entire waste water stream to boiling and volatile organic components are evaporated. Even with energy recovery, this process is very energy intensive.
Some contaminants can be removed by oxidation catalysis or UV ozonation. UV ozonation destroys the volatile organic contaminants by oxidation in the presence of UV light. This prevents recovery of the organics for reuse. Also UV irradiation can be harmful and dangerous (i.e., mutagenic).
U.S. Pat. No. 4,960,520 describes an absorption process to remove volatile organic contaminants from an aqueous solution. Contaminated water is pumped through hollow fiber membranes of microporous polypropylene having a very thin outer coating of plasma polymerized disiloxane (a silicone rubber). The hollow fibers are potted in a module resembling a shell and tube heat exchanger and strippant (oil) is pumped through the module shell. VOCs in the water diffuse across the membrane and dissolve into the oil. This process can remove contaminants. However, this process does not result in much VOC concentration and it cannot recycle the contaminants because the contaminants are now in an oil solution. In this configuration and process, it is necessary to have the membrane be as thin as possible to function only to separate the oil phase from the aqueous phase but allow contaminants to pass through as quickly as possible. One problem with this procedure and configuration is that oil can pass backwards through the membrane into the aqueous stream and contaminate it.
A similar method and apparatus is described in U.S. Pat. No. 4,915,838 for gaseous feed streams contaminated with organic vapors. Organic vapors are removed from ambient atmosphere by a thin microporous membrane medium together with a non-volatile collecting fluid having an affinity for the gaseous contaminant. Here, the membrane acts to stabilize the interface between the collecting fluid and air. The membrane is non-selective and acts to provide a large surface area for contact between air and the collecting fluid.
There are also often fouling problems associated with separating volatile components from feed streams. Depending upon the nature of the separation, fouling can be biological, mineral or organic. Biological fouling occurs when there is microbial contamination of an aqueous medium that creates a slime to foul the separating device. Mineral fouling is a problem in water treatment equipment when mineral deposits can foul surfaces and effect mass transfer. Organic fouling from non-volatile, low solubility organic substances can occur and interfere with equilibrium or membrane processes.
Another problem sometimes encountered in environmental remediation systems is that the ground water is contaminated by both volatile contaminants and non-volatile organic contaminants. Irrespective of energy used, both sorption and pervaporation processes cannot remove the non-volatile contaminants effectively without creating a solid waste disposal problem. Accordingly, there is a need to design better systems to remove multiple contaminants.
Still further, there is often contamination of aqueous feed streams with non-volatile contaminants (e.g., dioxin) without the presence of a contaminating volatile component. Even if the dioxin is removed, disposal is often as a hazardous solid waste, which can simply transfer the problem to another site. Therefore, there is a need to be able to remove non-volatile contaminants in a form so that they can be destroyed, such as by incineration.
Therefore, there is a need in the art to provide an energy efficient process for removing volatile components from feed streams. There are many applications where a volatile component must be separated from a non-aqueous solution. Further, there are many examples where water must be separated from organic solvents (i.e., when water is a dilute volatile component). Such applications and many similar applications require an energy-efficient method and device for removing and recovering dilute volatile components. The invention was made to address this need.